The invention relates generally to heat exchanger construction and more particularly to support plates for retaining tube array spacing within the heat exchanger.
The pressurized water vapor generators or heat exchangers, associated with nuclear power stations and which transfer the reactor-produced heat from the primary coolant to the secondary coolant that drives the plant turbines may be as long as 75 feet and have an outside diameter of about 12 feet. Within one of these heat exchangers, straight tubes through which the primary coolant flows may be no more than ⅝ inch in outside diameter, but have an effective length of as long as 52 feet between the tube-end mountings and the imposing faces of the tube sheets. Typically, there may be a bundle of more than 15,000 tubes in one of these heat exchangers. It is clear that there is a need to provide structural support for these tubes in the span between the tube sheet faces to ensure tube separation, adequate rigidity, and the like.
The tube support problem has led to the development of a drilled support plate structure of the type described in U.S. Pat. No. 4,120,350. This support system consists of an array of flat plates that is arranged in the heat exchanger with the planes of the individual plates lined transverse to the longitudinal axes of the tubes in the bundle. Holes or apertures are drilled and broached in each of the flat support plates to accommodate the tubes. Each aperture has at least three inwardly protruding members that restrain but do not all engage or contact the outer surface of the respective tube. Bights that are intermediate of these inwardly protruding members are formed in the individual support plate apertures when the tube associated therewith is lodged in place to establish secondary fluid flow through the plate. The inwardly protruding members terminate in arcs that define a circle of a diameter that is only slightly greater than the outside diameter of the associated tube. The broached support plates are made of SA-212 Gr.B, a carbon material, and may include tube free lanes with unblocked broached holes which detrimentally allow low steam quality secondary fluid flow to pass through the unblocked holes.
It has been found, after long periods of operation, that deposits consisting primarily of magnetite are formed at the tube support plates. These deposits block the bights formed between protruding members and thus cause undesirable increases in pressure drop which will in turn result in an increase in the secondary water level in the downcomer. If corrective actions are not taken, the rising water level could potentially flood the steam bleed ports and the main feed water nozzles and result in a malfunction of the steam bleeding and the main feed water systems.
Corrective actions such as power derating, chemical cleaning or water slap are costly. Moreover, the removal of deposits by chemical cleaning or water slap could damage the support plates.
Accordingly, there is a need for a tube support plate which minimizes pressure drop and deposit blockage while providing adequate structural strength.
The problems associated with the prior art tube support plates are largely overcome by the present invention which resorts to a stronger more corrosive resistant plate material such as stainless steel and by forming hourglass shaped tube holes in the support plates which minimize pressure drop by reducing local turbulence and are less likely to cause the deposition of magnetite and other particles on the surface of the support plates.
In view of the foregoing it will be seen that one aspect of the invention is to manufacture the tube support plates out of a stronger more corrosion resistant material such as stainless steel.
Another aspect of this invention is to have the protruding members of the broached holes terminate in flat lands.
A further aspect of the present invention is to provide hourglass shaped broached holes in the tube support plates.
These and other aspects of the present invention will be more fully understood after a review of the following description of the preferred embodiment along with the accompanying drawings.